I had a single clip and I divided in two. Then if I want to merge them into one clip the only way to do it that I found is the Undo command … that anyway is visible if the previous cut was done only few step before!! Otherwise you lost all successive editing 😞 I tried the Group function but if you use some kind of transition or other editing, the two clips are not see as a single one so the action involve only one clip ... I think that a merge function should be available as any even not professional editor allows (e.g. Pinnacle Studio).
Depending on what you've done to the separate parts of the original clip, you may want to output what you have as a new clip. Set the "Work Area Bars" to include the split clip sections. Then output a new clip after selecting the "Work Area Bar Only" box. Put the new edited clip back into the project. Be sure to make your output settings match the original.
However, a split is really just a marker in a clip. You can remove it.
Right-click on the half of the clip that follows the split and select Delete. (This will remove that segment without moving the positions of any other clips on your timeline.)
Then drag the end of the first segment of the clip and extend it to fill in the gap. The effect will be the same as if you'd never split the clip!
Depending on what edits/adjustments you want to retain, you can reverse the order by deleting the first part and dragging from the second.
(Note: My response is heavily plagiarized from a discussion between Steve Grisetti and Bill Hunt in 2010!)